LU93243B1

LU93243B1 - Mesoporous hydrogenated titanium dioxide

Info

Publication number: LU93243B1
Application number: LU93243A
Authority: LU
Inventors: Damien Lenoble; Olga Ishchenko
Original assignee: Luxembourg Inst Science & Tech List
Priority date: 2016-09-30
Filing date: 2016-09-30
Publication date: 2018-04-05
Also published as: EP3519089A1; EP3519089B1; WO2018060520A1; EP3519089C0

Abstract

The first object of the invention is directed to a process for depositing TiCte on a substrate, comprising the steps of (a) providing a substrate, (b) performing said deposition with TiCU and water by atomic layer deposition on said substrate; and (c) performing an annealing step. Said process is remarkable in that said step (b) is carried out at a temperature inferior to 50°C, and said step (c) is carried out at a temperature comprised between 450°C and 600°C in ambient air. The second object is directed to a photocatalyst comprising at least one layer of mesoporous hydrogenated TiOa on a substrate and obtainable by the process in accordance with the first object of the invention. The third object is directed to an use of said photocatalyst in a photocatalytic reaction performed under UV-visible light.

Description

MESOPOROUS HYDROGENATED TITANIUM DIOXIDE

Description

Technical field [0001] The invention is directed to a process involving atomic layer deposition in order to deposit mesoporous hydrogenated T1O2 and to the resulting photocatalyst, additionally to its use in photocatalytic applications.

Background art [0002] It is known that the major problem of T1O2 ALD synthesis from TiCL and water is the important roughness of the crystalline films (see Cheng H.-E., et at., Morphological and Photoelectrochemical properties of ALD T1O2 films, J. Eiectrochem. Soc., 2008, 155 {9), D604). Therefore it is mandatory to optimize the process growth in order to get the best trade-off between the film quality (that would trigger the photocatalytic properties) and its roughness that needs to be compatible with a 40 nm pore diameter.

[0003] Earlier research on T1O2 ALD deposition from TiCL and H2O determined typical deposition temperatures in a range of 200°C - 400°C (see Aarik J. et ai., Morphology and structure of T1O2 thin film grown by ALD, J. Cryst. Growth., 1995, 148(3), 268-275). However, the high reactivity of the precursor also enables film formation at lower temperature (see Tan L. K., et ai., Free-Standing Porous Anodic Alumina Templates for ALD of Highly Ordered T1O2 Nanotube Arrays an Various Substrate, 2008, 112(1), 69-73).

[0004] However, when porous substrates are used, deposition at low temperature is known to cause an inhomogeneous deposition into the porous space, leading to rough surface. This is a problem when further coating must be performed, for example for providing particular surface properties to substrate presenting a complexed surface.

[0005] Photocatalysis is a surface-dependant approach. Therefore, it is obvious that the increase of specific surface area would increase the photocatalytic activity. Thus, two kinds of nanoscale photocatalysts can be distinguished: suspensions of nanostructures in solutions or nanomaterials supported on appropriated templates. The main advantages of dispersed nanostructures are the simplicity of use and their low cost. Nevertheless, nanomaterials dispersed in solution tend to agglomerate, lowering their exposed specific surface area. Moreover, due to the toxicity of the nano-objects, they have to be removed after the pollutants degradation cycle, thus an additional filtration step is needed. For this reason, there is a growing interest in supported nanomaterials. To increase the exposed specific surface area of nano-textured photocatalysts, their synthesis at the surface of porous supports like membranes has been envisioned for water treatment (see Mozia S., Photocatalytic membrane reactors (PMRs) in water and wastewater treatment, Sep. Purif. Techno!., 2010, 73(2), 71-9!) or water splitting (see Tsydenov D.E. et ai, Toward the design of asymmetric photocatalytic membranes for H2 production: preparation of TiÖ2-based membranes and their properties, int. J. Hydrogen Energy, 2012, 37(15), 11046-11060).

[0006] The conventional photocatalytic approach, which uses basic metal oxides such as T1O2, ZnO, SnC>2, is far from reaching the required efficiency. Light absorption limited to the UV range and fast recombination of photogenerated carriers are the main bottlenecks.

[0007] Various solutions were proposed to overcome these limitations. The direct band gap engineering via doping intends to lower the band gap energy by creation of additional energy levels. It will extend the light absorption in the visible range. The assembly of metal-oxides with plasmonic nanoparticles also allows the activation of conventional photocatalysts in the visible range. Moreover, this approach improves the photocatalytic activity in the UV range by electrons trapping. The semiconductor/semiconductor heterostructures approach acts on the carrier separation. The light management approach is already applied in the field of photovoltaics; it would be also interesting for the photocatalytic applications as a way of light absorption improvement. The periodic organisation of metal oxide nanostructures with a high refractive index offers myriad possibilities of light manipulation.

[0008] Nevertheless, the methods allowing the metal oxide structuration in the periodic arrays at deep sub-wavelength scale are expensive, especially for T1O2 and SnC>2.

[0009] Therefore, it is an open challenge to develop large-scale and low-cost methods of periodic doped metal-oxides nanostructures over highly reflective substrate and to combine them with well- and periodically organised plasmonic nanoparticles.

Summary of invention

Technical Problem [0010] The invention has for technical problem to alleviate at least one of the drawbacks present in the prior art. More particularly, the invention has for technical problem to provide a simple process for considerably enhancing the photocatalytic properties of T1O2.

Technical solution [0011] The first object of the invention is directed to a process for depositing T1O2 on a substrate, comprising the steps of (a) providing a substrate, (b) performing said deposition with TiCU and water by atomic layer deposition on said substrate; and (c) performing an annealing step. Said process is remarkable in that said step (b) is carried out at a temperature inferior to 50°C, and said step (c) is carried out at a temperature comprised between 450°C and 600°C in ambient air.

[0012] According to a preferred embodiment, said step (b) is carried out at a temperature comprised between 20°C and 50°C.

[0013] According to a preferred embodiment, said substrate is curved glass.

[0014] According to a preferred embodiment, said substrate is porous.

[0015] According to a preferred embodiment, said substrate is a silicon substrate with a layer of silicon oxide.

[0016] According to a preferred embodiment, said step (b) comprises pulsing TiCU on said substrate so as to form a functionalized substrate, purging with an inert gas, pulsing H2O on said functionalized substrate, and purging with an inert gas, said inert gas being preferentially nitrogen gas.

[0017] According to a preferred embodiment, said purging with an inert gas lasts between 5 seconds and 30 seconds.

[0018] According to a preferred embodiment, step (c) is carried out for at least 2 hours.

[0019] According to a preferred embodiment, step (b) is performed between 100 times and 1000 times.

[0020] The second object of the invention is directed to a photocatalyst comprising at least one layer of mesoporous hydrogenated T1O2 on a substrate and obtainable by the process in accordance with the first object of the invention.

[0021] According to a preferred embodiment, said at least one layer of mesoporous hydrogenated T1O2 has a thickness which is comprised between 30 nm and 170 nm.

[0022] The third object of the invention is directed to an use of a photocatalyst as defined in accordance with the second object of the invention in a photocatalytic reaction performed under light irradiation, said light having a wavelength comprised between 1 nm and 780 nm.

[0023] According to a preferred embodiment, said photocatalytic reaction is a photocatalytic reaction of chemical degradation of acidic compound, neutral compound and/or basic compound.

[0024] According to a preferred embodiment, said photocatalytic reaction is a photocatalytic reaction of chemical degradation of methylene blue.

[0025] According to a preferred embodiment, said photocatalytic reaction is a photocatalytic reaction of chemical degradation of a mixture of methylene blue, rhodamine B and/or salicylic acid.

[0026] According to a preferred embodiment, said photocatalytic reaction is a photocatalytic reaction performed in water, wastewater, see water, river water, and/or industrial water.

[0027] According to a preferred embodiment, said photocatalytic reaction is a photocatalytic reaction performed in a water splitting process, a photolysis process and/or a hydrogen generation process.

[0028] In general, the particular embodiments of each object of the invention are also applicable to other objects of the invention. To the extent possible, each object of the invention is combinable with other objects.

Advantages of the invention [0029] The invention is particularly interesting in that it provides a way to obtain a photocatalyst of titanium oxide that can act under the action of UV-visible light (light with a wavelength comprised between 1 nm and 780 nm) to perform efficient chemical degradation of organic compound, acidic, basic or neutral. Such photocatalyst is also very stable to the reaction conditions and can be reused multiple times.

Brief description of the drawings [0030] Figure 1: Optical (SEM) images of T1O2 films deposited in temperature range of RT-400°C.

[0031] Figure 2: X-Ray Diffraction (XRD) T1O2 anatase (101) peak evolution as a function growth of temperature (200°C-400°C).

[0032] Figure 3: Diffractogram of the T1O2 film synthesized at 400°C.

[0033] Figure 4: Growth rate of ALD T1O2 films deposited at RT-400°C.

[0034] Figure 5: Possible paths of TiCU chemisorption on oxide surfaces.

[0035] Figure 6: QCM as a function of deposition time recorded profile at 200°C using purge time of 2 s, 5 s, 30 s and 600 s for both precursors.

[0036] Figure 7: QCM as a function of deposition time as a recorded profile at 100°C using purge time of 600 s for both precursors.

[0037] Figure 8: QCM as a function of deposition time as a recorded profile at 20°C using purge time of 1800 s for both precursors.

[0038] Figure 9: QCM as a function of deposition time as a recorded profile at 20°C using purge time of 5s and 30s for both precursors.

[0039] Figure 10: XRD of T1O2 deposited at 200°C after annealing at 600°C.

[0040] Figure 11: Photocatalytic degradation of methylene blue solution on the room temperature-deposited T1O2 films under visible light.

[0041] Figure 12: Photocatalytic degradation constant normalized by the sample surface area as the function of the film thickness for both purge time regimes, namely 5 s and 30 s.

[0042] Figure 13: XPS O 1s spectra of amorphous and annealed T1O2 film obtained by room temperature ALD.

[0043] Figure 14: Comparison plot of the absorption coefficient (left axis) and photoluminescence spectra (right axis).

[0044] Figure 15: Optical (SEM) images of room temperature T1O2 film deposited after 965 cycles in short purge time regime (top view and tilted images).

[0045] Figure 16: Repeatability of the methylene blue photocatalytic degradation in the UV range repeated four times.

[0046] Figure 17: Photocatalytic degradation of methylene blue, rhodamine B and salicylic acid on room temperature T1O2 films in the UV and visible ranges.

Description of an embodiment [0047] Thin films of -25-40 nm are obtained using standard conditions provided by the ALD equipment supplier: precursors pulse 0.2 s and purge time 2 s for both precursors (for temperature range 200-400°C).

[0048] In the low temperature range (20°C (RT) -200°C) depositions of thicker films (-100 nm) are obtained with longer purge time as 5 s and 30 s. The SEM and AFM measurement (see figure 1 and table 1) show that the roughness of films increases with the deposition temperature.

[0049] Table 1: Summary on the thickness and roughness of T1O2 films (as deposited)

[0050] Samples grown in the low temperature range (RT-200°C) show a smooth and highly conformal films, while films obtained at 350°C show the highest RMS value (16.98 nm).

[0051] It is worth noting that the roughness decreases suddenly for the films deposited at 400°C. These results until 350°C are in agreement with the literature (see Huang. Y, et at., Characterization of tow temperature deposited ALD TiO2 for MEMS application, J. Vac. Sci Technoi A Vacuum, Surfaces, Films, 2012, 31, 01A148). Therefore, from the morphological point of view, for further deposition into porous substrate, the low temperature range (RT-200°C) is privileged.

[0052] However, for better understanding of the morphology variation, the growth mechanisms have to be discussed. The formation of random nanoparticles for the deposition at 200°C and purge time 2 s has been observed, similarly reported by Luka et ai (Kinetics of anatase phase formation in Τ1Ό2 films during ALD and post-deposition annealing, CrystEngComm., 2013, 15(46), 9949). Such nanoparticles formation may unfortunately cause an inhomogeneous deposition into the porous space. The increase of the purge time to 5 and 30 s has solved this problem.

[0053] The XRD analysis demonstrates that films synthesised at a temperature below 300°C do not show any crystalline structure, therefore

diffractograms for films deposited at RT and 100°C are not presented (see figure 2). The anatase crystalline phase is detected for the depositions beyond 300°C and the increase of deposition temperature up to 400°C logically improves the film crystallinity (see figure 3).

[0054] The growth rate (growth per cycle, GPC) in whole temperature range (RT-400°C) represented on figure 4 for T1O2 films.

[0055] Three growth regimes can be distinguished: (i) low temperature regime (RT-200°C) where the growth rate decreases with the temperature increase that corresponds to the ALD condensation regime; (ii) intermediate regime (200°C-350°C) demonstrate a growth rate increase followed by the standard ALD plateau (iii) at high temperature regime (>350°C).

[0056] It is generally established that the surface reaction with the metal-oxide precursor mainly involves the OH functional groups (see Aarik J. et al, ALD of T1O2 from T1CI4 and H2O: investigation of growth mechanism, Appi. Surf. Sci., 2001, 172, 148-158). Therefore, the typical T1O2 growth reaction follows equations 1 and 2 and is schematically represented on Figure 5.

[0057] [Pulse TiCL]: x(-OH)x + TiCL- (-O-)xTiCL-x + xHCI (x=1, 2) (equation 1)

[0058] [Pulse H2O]: (-O-)xTiCL-x + (4 - x)H2O - (-O-)xTi(OH)4-x + (4-x)HCI (equation 2) [0059] Nevertheless, the surface chemistry is highly dependent on the temperature. At low temperatures (<300°C), the isolated hydroxyl groups OH and H-bonded OH groups are dominate on the surface of substrates such as S1O2 or AI2O3. The increase of temperature (>300°C) may lead to the dehydroxilation and the formation of oxygen bridges (equation 3).

[0060] Ti(-OH)-O-Ti(OH) -Ti(-O-)2Ti +H2O (equation 3) [0061] At high temperature, the oxygen bridge terminated surface promotes also the direct TiCL chemisorption according to lla-llb reaction paths showed on figure 5 and used to improve the film quality because of the lack of HCI by-product.

[0062] The GPC increase in the intermediate regime can be related to the temperature dependent surface functionalities and also the important role of the HCI by-product on the growth mechanisms.

[0063] In the literature, the role of HCI on the T1O2 growth mechanism was fully established by Leem eta/., Role of HCI in ALD of T1O2 thin films from TiCU and water, 2014, 35(4), 1195-1201). The authors showed that in the temperature range 150-300°C, the volontary addition of the HCI after the TiCU pulse significantly lowers the growth rate. For the high temperature range (>300°C), the additional HCI increased the growth rate. The authors postulated that at lower temperatures (150-300°C), the surface reaction occurs via OH-groups and forms intermediate or non-stable products, according to the reactions depicted on equations 1 and 2. HCI may induce the reversible reaction and promote the desorption of TiCU-x or Ti(OH)2Cl2 volatile compounds. The overall reaction being partially reversible, the growth rate of T1O2 is lowered. Beyond 300°C, the oxygen bridge terminated surfaces are induced and promote the TiCU adsorption as previously described without any release of HCI. The oxygen bridges are mainly triggering the T1O2 growth reaction according to equation 4.

[0064] Ti(-O-)2Ti +TÎCU -* Ti(CI)-O-Ti(OTiCI3) (equation 4) [0065] This investigation highlights that the mono- (figure 5 la) or bifunctional (Figure 5 lb) absorption of TiCU on hydroxyl groups is reversible under HCI atmosphere, while dissociative absorption on oxygen bridges is irreversible. Interestingly, while HCI was added at the end of the ALD cycle (after water pulse), it significantly decreases the growth rate for the whole studied temperature range (150 - 400°C). When oxygen bridges are exposed to HCI (equation 5 and the schematic lllb-lllc represented on figure 5). T1O2 surfaces are functionalised with -OH, -Cl, =OHCI. That retards the growth of T1O2. This fundamental investigation highlights the predominant role of HCI on the T1O2 growth mechanism. The time of residence of HCI in our ALD processes is strongly dependent on the purge time.

[0066] Ti(-O-)2Ti + HCl -+ Ti(CI)-O-Ti(OH) (equation 5) [0067] Therefore, further investigations require the optimisation of purge time using the quartz crystal microbalance (QCM). The QCM being integrated into the reactor lid cannot be used at deposition temperature above 250°C. Henceforth, the low temperature TiO2 deposition regime was principally considered: at RT, 100 and 200°C, so more as these regimes conduct to the low surface roughness of TiO2 films.

[0068] The importance of the purge time is logically more significant in the low temperature regime, where the excess of precursors and by-products desorption is not assisted by heat.

[0069] The QCM measurements realised first at 200°C using pulse time 0.2 s and purge time 2, 5, 30 s for both precursors (see figure 6). The pulse of TiCU induces the mass increase following stabilisation during the purge.

[0070] The addition of a water pulse enables the surface reaction and the mass drop due to the release of HCI by-product (figure 6B). The completed mass stabilisation was not achieved in the case of a 2 s purge time.

[0071] The increase of the purge time up to 5 (figure 6C) and 30 s (figure 6D) improved the mass stabilisation. However, for a purge time of 30 s after the TiCU pulse, an anomalous mass drop after 12-15 s of purge time is observed (figure 6D). To further study the “relaxation” time after the pulse of each precursor, an exaggerated purge of 600 s is applied (figure 6E).

[0072] This study shows that after the TiCU pulse the mass signal decreases continuously during the first 5-8 s of purging, but then the mass reincreases significantly without any visible stabilisation. The water pulse promotes a slow mass decrease within the first 10 s of purging, then the sensor signal increases again.

[0073] Such QCM behaviour of mass increases within the long purge time is intriguing. It may be associated to the re-adsorption of volatile species or surface reaction by-products from the reactor walls and re-deposits on the QCM surface.

[0074] Moreover, after the completed 50 cycles deposition with purge time 2 and 5 s, the mass sensor signal continues to decrease; that could be characteristic of the desorption of volatile compounds (Figure 6 A zoom).

[0075] Aarik and co-workers (Aarik J. et al, ALD of T1O2 from TiCU and H2O: investigation of growth mechanism, Appi. Surf. Sci., 2001, 172, 148-158) also pointed out on the continuous decrease of mass sensor signal during the purge time for depositions realised at 100°C. The authors have suggested the continuous dehydration of the surface after exposure to water pulse at low temperature, or desorption/decomposition of formed Ti(OH)xCl4-x volatile species.

[0076] The QCM control on the mass evolution during the ALD deposition demonstrated that, at 200°C, the purge time after the water injection should be longer than 600 s.

[0077] The decrease in the deposition temperature to 100°C does not significantly modify the mass sensor profiles compared to the previously recorded for 200°C. The study of the relaxation time at 100°C demonstrates that a purge time of 600s is still not sufficient to stabilise the mass sensor signal after the pulse of TiCU, however after injection of water, the mass sensor signal stabilisation is interestingly achieved after a purge time of 30 s of purge (see figure 7).

[0078] The mass change, as recorded by QCM, for depositions realised at room temperature is significantly different from previously discussed QCM profiles recorded for thin-films grown at 100 and 200°C. An anomalous behaviour is noted while the water pulse is supplied. Instead of the mass decrease due to the HCI release, the mass sensor shows a mass increase and then a continuous decrease of the mass increment without stabilisation, even after 30 min of purge (figure 8).

[0079] The mass increment per cycle is strongly dependent on the purge time. The mass increments per cycle of ~ 0.14, 0.06, and 0.02 pg/cm2 is determined for depositions realised with 5, 30 and 1800 s purge time. The final mass increment, after 50 cycles, is twice larger for 5 s purge time than for 30 s purge time (see figure 9) at room temperature.

[0080] For high temperature (100 and 200°C), the final mass increment is not significantly different for the different purge time. This demonstrates that the unreacted precursors or by-product thus accumulate within the film at room temperature. At a low temperature, the surface is mainly functionalised by hydroxyl groups and H-bonded OH. Furthermore at temperatures below 100°C, the presence of molecularly absorbed water on the surface is inevitable. Thus, TiCL molecules could react not only with functional groups but also with residual water. The reaction of TiCL hydrolysis at room temperature generally leads to the formation of TiOCL, TiOCI, T12O3 and/or Ti(OH)xCL-x with generation of HCI. All of these chemistries may be expected in the films.

[0081] Several theoretical studies have reported that the first half-reaction of TiCL and H2O is endothermic and thus the desorption of HCI is hampered. That is in line with our hypothesis about an accumulative growth mechanism at room temperature.

[0082] Nevertheless, it is important to consider the slow film growth rate (~6 Â/ cycle for purge 30 s) for which a further increase in the purge time (to reach the ideal ALD regime) will lead to an unreasonably long deposition time (dozens of hours), for the considered T1O2 thickness.

[0083] Consequently, T1O2 thin-films deposited with purge times of 5 s and 30 s were chosen and since ALD thin films grown at low temperature are amorphous, the post-deposition annealing for engineering the thin film crystallisation was used. The use of anodized aluminium oxide (AAO) membranes for further T1O2 nanowires fabrication also imposes the annealing temperature limit at 600°C.

[0084] The used ALD pulse/purge parameters are 0.2 s pulse /5s and 30 s nitrogen purge for both precursors and result in different thicknesses of 25, 50, 90 nm corresponding to 500, 1000, 2000 cycles. The T1O2 films grown with the purge time 30s are much less crystallised than the films grown with 5 s purge time (figure 10).

[0085] The ALD process is thus carried out at a temperature inferior to 50°C, preferentially at a temperature comprised between 20°C (room temperature) and 50°C.

[0086] The photocatalytic activity of the room temperature-deposited TiO2 films with various thickness and grown with both purge time is tested under the visible range irradiation. The obtained degradation curves on methylene blue, presented on figure 11, confirm the activity of the films in the visible range whatever the used growth process, substrate and thickness.

[0087] The best performing mesoporous thin film is grown on thermally oxidised Si substrates. The photocatalytic degradation is enhanced with the film thickness, with a degradation rate constant reaching 57.6*104 for the 170 nm thick film grown with 5 s purge time. In the visible range, the photocatalytic degradation constant normalised by the sample surface area versus film thickness is higher in the case of films deposited on Si with a 30 s purge time (figure 12). For the short purge time (5 s), thin films lower than 30 nm are not active in the visible range when grown on the Si substrate. Only thicker film (170 nm) shows degradation when the 5 s purge time process is used to elaborate the mesoporous TiO2. However, the deposition of a thick film of 170 nm on the oxidised substrate (Si/SiO2) with 5 s purge time shows the maximum photocatalytic degradation values at 8.0*104 min-1cnr2.

[0088] The interplay between physical and chemical properties of room temperature ALD TiO2 films and their photocatalytic performance in both UV and visible ranges is discussed hereafter.

[0089] The use of the room temperature process condition significantly modifies the TiO2 thin film chemistry and morphology. First of all, an amorphous thin-film of TixOy is grown and traps unreacted precursors, reaction byproducts, and complex Ti-Ox-Hy-Clz chemistries typically characterised as-grown thin films.

[0090] Then, the post-deposition annealing of the amorphous films enables the thermal oxidation of the film under ambient air conditions, promotes degassing of the chlorine compounds and leads to a mesoporous hydrogenated-TixOy thin film.

[0091] The degassing of a chlorine compound (typically HCI or Cl2) may induce oxygen vacancies in the film volume and Ti3+. The chemical characterisation of the films before and after annealing by the XPS analysis does not show any change in the Ti 2p spectra. O 1s peak is systematically shifted towards high binding energy (+0.3 eV) when amorphous films are annealed (see figure 13). O 1s shoulder at 532 eV is attributed to the hydroxylation of T1O2 (Ti-OH) (see Chen X. et at., Increasing solar absorption for photocataiysis with black hydrogenated ΠΟ2 nanocrystais, Sciences, 2011, 331 (6018), 746-750).

[0092] Chen et ai. noted two O 1s peak contributions at 530 to 530.9 eV without Ti 2p spectral modification. The authors postulated that hydroxylation of TiO2 by hydrogen treatment induces a significant disorder of the crystalline structure i.e. oxygen vacancies, hydroxylation of the dangling bonds and Ti-OH.

[0093] These structural defects may have a significant impact on the electronic properties via inter bang gap energetic levels. The theoretical predictions realised by Chen et ai. predicts that disordered hydrogenated-TiO2 crystals induce mid-gap electronic states at 1.8 eV and high-energy states at 3 eV, leading to a band gap narrowing.

[0094] In the present case, photoluminescence (PL) profiles (see figure 14) evidence defects contributing to the radiative recombination of carriers at -500-525 nm (2.48-2.36 eV), ~585nm (2.11 eV) and ~700nm (1.77 eV).

[0095] The room temperature TÎO2 samples (965 cycles, 5s) deposited on quartz substrates are characterized by UV-Vis spectroscopy. The optical band gap remains close to usual T1O2 values (-3.1 eV) however an additional absorption band is noted between 2 and 3 eV (413-620 nm). This corresponds to the light absorption in the visible range. PL peaks are also reported for comparison and evidences a lower radiative energy of excitons than the peak energy of the visible absorption. The atypical stoichiometry and morphology of room temperature thin-films induces multiple energy levels within the T1O2 band gap.

[0096] From these first results, it was suggested that room temperature thin-films feature specific oxygen vacancies and Ti-OH coordination. Room temperature ALD process thus leads to HCI or Ti-Ox-Hy-Clz in amorphous films.

[0097] During annealing, the HCI degasing promotes oxygen vacancies and Ti3+ while its thermals decomposition at 600°C into Cb and H2 leads to “in-sitd' hydrogen treatment of T1O2.

[0098] Hydrogenation process has been shown recently to modify significantly the opto-electronic properties of T1O2 due to a significant concentration of Ti-OH inducing disordered crystalline arrangement. Such features are shown to modify considerably the photocatalytic responses (see Park S.K et al, Effect of HCI and H2SO4 treatment of TiO2 powder on the photosensitized degradation of aqueous rhodamine B under visible light, J. Nanosci. Nanotechnol, 2014, 14(10), 8122-8128).

[0099] In the present case, the astonishing photocatalytic efficiency is also correlated to the immense specific area (see optical images on figure 15) and the “in-sitü’ hydrogenation of T1O2.

[00100] The samples having a very high surface area, the lack of simple adsorption of methylene blue (i.e., without any degradation) within the sample should be checked. To this end the photocatalytic degradation is repeated 4 times on the same sample. Moreover, for one of the runs, the sample is held overnight in the methylene blue solution. In that case, the change in the methylene blue concentration is almost insignificant. The photocatalytic degradation rate is unchanged even after 4 runs, highlighting the very high stability and efficacy of the samples that show unchanged photocatalytic performance overtime.

[00101] After four hours, methylene blue, an acidic molecule, is fully degraded, regardless of the number of experiments performed with the same sample. This remarkable photocatalytic efficiency competes with the very best material solution provided today (see figure 16).

[00102] The methylene blue affinity to the T1O2 surface facilitates its degradation. An efficient photocatalytic system shall degrade multiple molecule chemistries. Therefore, the mesoporous T1O2 is also exposed to other target molecules such as rhodamine B (a basic molecule) and salicylic acid (a neutral molecule) at the same time as methylene blue.

[00103] Those photocatalytic tests are performed using methylene blue, rhodamine B, and salicylic acid solutions with an equivalent concentration of 5 mg/L under UV and visible irradiation (see figure 17). The molar concentration for methylene blue, rhodamine B, and salicylic acid corresponds to 15, 10 and 36 pmol/L, respectively. The degradation rate in the UV range, namely with a light with a wavelength comprised between 1 nm and 400 nm, is almost identical for the three target molecules.

[00104] Table 2 indicates the photocatalytic degradation constants on room temperature TiO2 normalized by the sample surface area (min1 cm 2), and photolysis constants of the reference solutions.

[00105] In the present case, the photocatalytic degradation under UV light is not dependent on the molecular affinity towards of the photocatalyst surface. This feature is of very high interest for depolluting water typically characterised by a mixture of pollutant molecules having significantly different chemical features such as steric volume, polarity, and chemical termination.

[00106] Water can be sea water, river water, wastewater and/or industrial water. [00107] The photocatalyst can also be used in a water splitting process, a photolysis process and/or a hydrogen generation process.

[00108] For instance, when considering emerging photocatalysts such as the Bi2WC>6 nanosheet with a positively charged surface, the methylene blue and rhodamine B were found to have higher adsorption, while neutrally-charged molecule like salicylic acid are hardly adsorbed (and thus degraded) on such a surface (see Zhou Y. et at., Monolayered EfoWOe

nanosheets mimicking heterojunction interface with open surfaces for photocataiysis, Nat. Comm., 2015, 6, 8340).

[00109] In the present case, a slight increase in the degradation constant on salicylic acid is measured when compared to the two other molecules.

[00110] When visible light irradiation is used, namely a light with a wavelength comprised between 400 nm and 780 nm, the degradation rate for the same tested pollutants is different. The degradation constant of methylene blue is the highest, but the “natural” degradation of methylene blue under visible light must be considered, as evidenced by the significant photodegradation of the reference solution. The salicylic acid and rhodamine B molecules are highly stable under visible light, as proven by the concentration plateau of the reference solution. The photocatalytic degradation under visible light is even more efficient for neutral molecules such as salicylic acid compared to methylene blue and rhodamine B, showing that the mesoporous T1O2 material is particularly efficient. The photocatalytic degradation rate for different molecules was formerly explained by better adsorption due to the charge difference between the surface and the molecule.

[00111] However, those results in the UV range contradict this assumption, showing an identical degradation rate for all molecules. The lower activity in the visible range may be due to the lower carrier density and thus lower radical concentration. Therefore, in the visible range, the degradation occurs according to the stability of the molecules.

Claims

A method for deposition of T1O2 on a substrate, comprising the steps of: a) providing a substrate; b) performing said deposition with TiCU and water by atomic layer deposition on said substrate; and c) performing an annealing step; wherein step (b) is carried out at a temperature below 50 ° C, preferably at a temperature of between 20 ° C and 50 ° C, and wherein step (c) is carried out at a temperature between 450 ° C and 600 ° C in ambient air.

The method of claim 1, wherein said substrate is curved glass.

The method of claim 1 and / or claim 2, wherein said substrate is porous.

The method of any one of claims 1-3, wherein said substrate is a silicon substrate of a silicon oxide layer.

The method according to any one of claims 1-4, wherein said step (b) is to draw TiCU on said substrate to form a functionalized substrate, to be purged with an inert gas, to draw H2O on said functionalized substrate and purging with an inert gas, said inert gas preferably being nitrogen gas.

The method of claim 5, wherein said purging with an inert gas lasts between 5 seconds and 30 seconds.

The method of any one of claims 1-6, wherein step (c) is performed for at least 2 hours.

The method of any one of claims 1-7, wherein step (b) is performed between 100 times and 1000 times.

Photocatalyst comprising at least one layer of mesoporous hydrogenated ΤΊΟ2 on a substrate and obtainable by the method according to any one of claims 1-8.

10. Photocatalyst according to claim 9, characterized in that said at least one layer of hydrogenated mesoporous T1O2 has a thickness which is between 30 nm and 170 nm.

11. Use of a photocatalyst as defined in any one of claims 9-10 in a photocatalytic reaction carried out under irradiation with light, said light having a wavelength of between 1 nm and 780 nm.

12. Use according to claim 11, in a photocatalytic reaction of the chemical degradation of an acid compound, a neutral compound and / or a basic compound.

13. Use according to claim 11, in a photocatalytic reaction of the chemical degradation of methylene blue.

14. Use according to claim 11, in a photocatalytic reaction of the chemical degradation of a mixture of methylene blue, rhodamine B and / or salicylic acid.

15. Use according to any one of claims 11-14, in a photocatalytic reaction carried out in water, waste water, sea water, river water, and / or water. industrial water.

16. Use according to any one of claims 11-15, in a photocatalytic reaction carried out in a water separation process, a photolysis process and / or a hydrogen generation process.